The invention relates to screen stencil exposure apparatus.
The process of producing stencils on screens for use in stencil screen printing typically involves coating a chase mounted screen with a photo responsive emulsion, drying the coating, overlaying the coated fabric on a film positive, exposing pattern areas of the emulsion coating to light through the stencil pattern positive and subsequently developing the exposed emulsion to cause openings in the screen in the pattern desired. This prepared screen is used in the printing of conventional serigraphs by forcing stenciling fluid or ink through the openings to selectively coat the underlying stock.
Conventional apparatus for making these stencils normally includes a translucent, and preferably a transparent, surface on which the positive is mounted. A chase on which the emulsion coated screen is stretched is placed against the translucent surface and stencil positive, with the periphery of the area to be exposed sealed between the screen and transparent surface. Suction is typically used to draw the screen and the positive together in order to produce intimate contact and therefore accurate exposure on the screen.
Heretofore, various techniques have been used to produce the peripheral seal between screen and translucent surface. One such technique simply involves manually placing a peripheral rubber tube around the stencil area and then pressing the screen chase against the transparent surface. This manual positioning is time consuming and requires manual placement each time a new screen is exposed. Additionally, such manual mounting normally requires a generally horizontal work surface in order to support the tube.
In another method a peripheral groove is recessed into the support platform around the translucent surface. A rubber tube is mounted within this groove and therefore manual placement is obviated. However, the size of the space so encompassed cannot be varied so that the stencil area is limited to substantially one size.
One method of sealing a vertical exposure unit conventionally includes the placement of a rubber blanket over the apparatus and drawing a vacuum thereon. This method is cumbersome and not very versatile. Another successful method of providing the peripheral seal for both horizontal and vertical exposure units that does allow some variation in the stencil area involves the mounting of an endless resilient tube onto the support surface for the translucent surface. The tube is mounted by four corner brackets that are movable to a variety of mounting positions on the support surface, while either the resiliency of the tube or a change in tube size accommodates the reduction in stencil area. Although this method does provide for some variation of the stencil area, the variation is limited by the adaptability of the tubular seal.